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rosweed

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I'm about to build my first green house. Has anyone had any experience with the walipini or underground greenhouse?

mattie

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Hello
I have seen the walipini on permaculture forums before they do make sense from the point of view of saving on building materials and costs, all you need to do is hire a digger and get some polythene and roof framing.
I have to wonder if the majority of the passive solar heat captured will simply be lost to the internal subsoil ,that said the temperature will remain stable year round,regardless of hot or cold temperatures at the surface.There is also some soil temperature maps that may be worth looking at for your region.

There is an issue with them i see connected to trench safety, if a wall decides to move or a chunk of soil to fall its a lot of weight and is actually quite dangerous.This may be reduced with some sort of bracing or supports.

Heres a video that may interest you



I would also consider a double layer of polythene with perhaps an air blower in between the layers of plastic to keep the energy inside the greenhouse when it gets cooler outside.
On that same channel there is a series of clips showing the complete build of the greenhouse.There is also some other posts on the forum regarding greenouses that would be worth looking at .
Regards Mattie
kcl1s

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I constructed an 8 x 12 solar greenhouse in Maryland the 90's. Floor level 4 ft. below grade. Excavated to 6 ft. put in 2" blue foam bottom and sides then 4 rows 4" perforated drain pipe connected to a 250 cfm blower in the peak, next 2 ft. (about 7 tons) 2" stone. I had 10 black 55 gal drums of water that supported my benches. North wall above grade had 4" insulation. My only vent was a 4 sq. ft. solar powered convection vent at the peak, assuming that all the mass would absorb most of the heat. No heat was installed.

I finished it in late fall. My only recording device was a hi/low mercury thermometer at the growing level. Here are the highs and lows by month

December        35 94 
January           33 92  
February          37 96
March              41 104 


I had lettuces, radishes, herbs, and started tomato plants. None of the plants did as well as I would have liked but none froze.

Now I operate a one acre commercial greenhouse growing spring annuals with 22 one hp. exhaust fans and burn $30,000 of propane in a season.

So here are my observations/recommendations. 

-I gave up the buffering advantage of the soil by insulating the floor and walls. A better choice would be to insulate the surface of the ground 4 ft. out from the edge of the building.

-Do not skimp on venting capacity.

-In my climate it is unlikely you can grow summer crops successfully in a passive greenhouse without supplemental heat and supplemental light for some plants.

-Supplemental heat should be utilized at the root zone and just above the soil surface creating a microclimate during the night. This could be accomplished with a modest solar water heat collector and some distribution loops of pex in the soil and fintubes at the plant canopy level.

Good luck on your build
Keith Lohmeyer

 
solardan1959

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Keith,
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In my climate it is unlikely you can grow summer crops successfully in a passive greenhouse

Are we still talking Maryland or some place cooler?

Quote:
None of the plants did as well as I would have liked
December        35 94 
January           33 92  
February          37 96
March              41 104


At what point do you think it was better? Was it general cooler daytime temps or the 33-37 that was too cool for good plant growth?

And a generic question for everybody, It's been much to cool in the sunspace, 11 degrees this morning though with sun today I'll see 80ish in there during the day.  Do you think a passive heated drum from the daytime temps would make it a couple of days with out freezing with no insulation on the drum?

If I added a small 2X6 water heater collector to help warm the water but wanted to circulate that water as suggested by Keith through the dirt, would I be better using a insulated drum or a collector and sun warmed drum as mention above?  The collector would be drain back.

I guess I should but the drum in there and fill it up with water to find out how it does.  Probably work better in a batch heater type box with glazing on the front. (actually that sounds like a good idea)

Dan
mattie

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Hello kcl1s
When you say a better choice would have been to insulate the surface of the ground 4 ft out from the edge of the building could you elaborate a bit more on this?

Another route to take like kcl1s said would be to insulate the exterior, perhaps straw would be a good cheap choice.Although you will have to make sure that no moisture makes contact with it by shielding it somehow,done cheaply polythene perhaps, maybe split into wall and floor section into panels and wrap each completely to make sure it is waterproof(there may be some condensation issues here )

The the red arrow here could be soil or barrels filled with water to add some thermal mass,the other arrow is the straw insulated exterior layer

Regards Mattie

BASIC WALAPINI.png 

kcl1s

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Dan,
I still live in Maryland. I think the poor performance was due to low soil temperature although I never took soil temps back then. If you can get your soil temps to 60 deg. and keep the foliage from freezing or burning (between 40 to 95 deg.) I think most plants will grow well.

If your drum is in the sunspace it is probably not necessary to insulate. To get useful heat for the soil you will need a collector of some type. My choice would be a collector outside the sunspace with storage tank inside. although a run of swimming pool collector mounted high on the back wall of the sunspace would be a cost effective option.

Keith 
kcl1s

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Reply with quote  #7 
Mattie,
Soil temps 3 ft. below grade here in Maryland stay in the 50 degree range year round. The reason for sinking the greenhouse in the ground is to take advantage of constant soil temp and the thermal mass to keep from over heating. I agree the soil is not efficient for high temp storage but that is not what you want from it in the greenhouse. You want to insulate from the cold first 2 ft. of ground. You can insulate vertically (the sides of the dug out soil) or you can insulate horizontally (the soil surface around the outside edge of the greenhouse creating a large insulated umbrella) allowing you to have more surface area for heat exchange to and from the soil thermal mass. Sloping the grade away from the greenhouse and poly to keep the soil dry are great ideas.

Keith
heatloss.png

mattie

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Reply with quote  #8 
Solardan1959 i think it would be interesting to see if there were a simple way to regulate the flow of heat (at night and early morning)from the water barrel after it has captured the solar gains during the day.The flow of heat will be dependant of the difference in temperature between hot(or maybe low is a better discription) and the colder outside temperature.So the lower the outside temp the quicker you will lose your stored thermal energy.
Regards Mattie
mattie

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Heres a guide i found online for anyone interested in building a walipini
http://www.bensoninstitute.org/Publication/Manuals/Walipini.pdf

mattie

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Im thinking solardan it may be a case of trying to control how airtight(not completely) and well insulated your sunspace is.In order to keep the solar energy trapped once it enters the structure.
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